1. Field of the Invention
The present invention relates to a process for the production of a directional accelerometer having a sensitive axis parallel to the plane of the substrate and/or the production of a directional accelerometer having a sensitive axis perpendicular to said plane, using silicon on insulator technology.
2. Discussion of the Background
An accelerometer is an acceleration transducer essentially having a moving mass m and means for measuring the force F=m.gamma. due to the acceleration .gamma. of a moving body on which the accelerometer is installed.
Nowadays acceleration is a parameter which is becoming increasingly useful and necessary to know for industrial requirements, particularly in the space and aeronautical fields, but also for applications in the motor vehicle sector for checking or controlling the active suspension or air bags. The development of the latter uses requires a very significant reduction in the production costs, while ensuring that the transducers still have acceptable metrological qualities.
In general terms, the accelerometers according to the invention can be used in all fields where it is desired to measure the acceleration of a body moving in one or two directions parallel to the substrate and/or one direction perpendicular to said substrate.
Numerous methods have been proposed for producing accelerometers or mechanical structures for accelerometers from micromachined silicon using microelectronics technologies.
The main advantage of silicon is clearly the collective treatment of the structures and their miniaturization, i.e. a relatively low cost price, but also the mechanical reliability the monocrystalline material which is not subject to creep, hysteresis or time drift. This significant cost reduction makes it possible to have an even broader use of such transducers or sensors, while maintaining acceptable metrological qualities.
There are two main groups of acceleration transducers, as a function of the position of the sensitive axis with respect to the semiconductor substrate. These are structures having a sensitive axis perpendicular to the substrate (called perpendicular axis structures hereinafter), which are the most widely developed and which use conventional silicon volume anisotropic chemical machining technologies, i.e. the entire thickness of the substrate is etched in order to free a monocrystalline structure. The structures having a sensitive axis parallel to the substrate (hereinafter called parallel axis structures), have the main advantage of integrating onto the same chip accelerometers sensitive to two coplanar axes and which may be perpendicular, said structures using a surface or volume technology.
Reference 1--Sensors and actuators, A21-A23 (1990), pp 297-302, "Precision accelerometers with .mu.g resolution" by R. Rudolf et al describes the production of a perpendicular axis accelerometer using this volume method.
A parallel axis transducer using the volume method is described in reference 2--Transducers' 91 Digest of Technical papers, June 1991, San Francisco, "A simple, high performance piezoresistive accelerometer" by J. T. Suminto, pp 104-107 and in reference 3--U.S. Pat. No. 4,653,326, filed in the name of the applicant.
The major disadvantages of a volume technology are the use of a double face method (few, expensive specific machines and substrates polished on both faces); a transducer shape linked with the crystalline orientation of the substrate and therefore a shape limitation; a miniaturization of the transducer limited by the substrate thickness (three-dimensional structure, one dimension being fixed); and the need to bring about a bonding of the transducer on one or more substrates requiring the use of structure support and reference cavities, which somewhat complicates the manufacture of these transducers.
In general terms, the basic principle used in silicon acceleration transducers is the measurement of the displacement or the force exerted by a seismic mass attached to a support by a flexible mechanical link, called flexible beam.
The vital quality of an accelerometer is its directivity. The latter is obtained by shape anisotropy of the mass support beams. These rectangular, varyingly long beams have a considerable flexibility along their thickness and a significant rigidity along their width. The control of the thickness of the beams, which will determine the transducer sensitivity, is the main difficulty in manufacturing the transducer.
For reasons connected with the mechanical resistance and the electrical characteristics, it is very important for the supports and the freed structure (mass) to be of monocrystalline silicon (due to the absence of creep, hysteresis, elasticity and the possibility of fitting electronic components).
In the case of a parallel axis transducer, the shape of the beams is obtained by etching a silicon substrate having a 110 orientation, for which the slow (111) etching planes are perpendicular to the substrate plane (cf. reference 2). In this case, it is a question of anisotropic etching making it possible to obtain a good geometrical definition, but which leads to a limitation of shapes in accordance with the crystalline orientation of the substrate. It also involves special substrates (polished on two opposite faces) and the use of a double face alignment method. In addition, these non-standard microelectronics substrates prevent integration on the same substrate of the associated electronics.
The significant thickness of the silicon substrate to be etched (approximately 500 .mu.m) requires the use of very selective etching masks and relatively large final etched patterns not permitting significant miniaturization.
When the sensitive element is finished, it is then necessary to bond it onto one or more thick, rigid supports to obtain a transducer. Said support or supports are generally of a different nature from that of the substrate (e.g. glass), which leads to differential stresses prejudicial to the performance characteristics of the transducer and to a difficult supplementary stage.
Other methods have been proposed for producing parallel axis transducers in single face technology (with all the stages on the front face) using a sacrificial layer with very small geometrical shapes and a freedom with respect to the latter. They also offer the possibility of integrating two parallel axis transducers on the sane substrate.
Thus, a parallel axis acceleration transducer with its integrated electronics has recently been marketed according to a surface machining method using a sacrificial layer and a polycrystalline silicon deposited layer forming the desired mechanical structure. This transducer is described in reference 4--Electronic Design, August 1991, pp 45-56, "Accelerometer's micromachined mass "moves" in plane of IC; on-chip circuit controls it and senses G with force-balance techniques" by F. Goodenough.
The main limitations of this surface method are the mediocre mechanical qualities of the polycrystalline material and the significant differential thermal stresses induced by the use of two different materials (monocrystalline Si substrate and polycrystalline Si structure), which lead to transducers having limited or even inadequate metrological qualities, as well as a thickness limited to a few micrometers for the polycrystalline silicon layer forming the seismic mass, which reduces the directivity, the dimensioning possibilities and the measurement ranges of the transducer.
In addition, the mobile polycrystalline silicon mechanical structure leads to a reduction in the metrological, reproducibility and stability characteristics of the transducer.
In addition, this parallel axis transducer has a shape ratio for the beams (height/width close to 1) which is not very advantageous from the directivity standpoint and therefore has a significant sensitivity to transverse accelerations. It is the use of polycrystalline silicon, whose deposition thickness does not exceed a few micrometers (generally&lt;2 .mu.m), which limits the directivity and size of the seismic mass and consequently the measurement ranges.
Moreover, for the production of perpendicular axis silicon acceleration transducers, the seismic mass supports are very frequently produced by an etch stop method. Use is then made either of the etching of a solid silicon substrate by the rear face with a stop on a highly boron doped epitaxied silicon layer (cf. reference 5--J. Electrochem. Soc., vol. 137, No. 11, November 1990, "Anisotropic etching of crystalline silicon in alkaline solutions: II. Influence of dopants" by H. Seidel et al, pp 3626-3632) or an electrochemical etching of the silicon substrate with etch stop on an epitaxied silicon layer forming a P/N junction with the substrate (cf. reference 6--IEEE Transactions on Electron Devices, vol. 36, No. 4, April 1989, "Study of electrochemical etch-stop for high-precision thickness control of silicon membranes" by B. Kloeck et al, pp 663-669).
These two etch stop methods suffer from the disadvantages referred to hereinbefore. Thus, they use an anisotropic etching of the substrate, limiting the shapes of the sensitive elements due to the crystalline orientation of the substrate, as well as an etching from the rear face requiring the use of special substrates and a double face alignment method.
Moreover, these stop methods require the use of highly selective etching masks and, bearing in mind the inclined etching planes (54.7% for orientation 100 silicon) and the thickness of the silicon to be etched, the shapes produced on the rear face greatly exceed the final useful shapes of the component.
Other methods have been proposed for producing perpendicular axis transducers employing surface technology. These methods are more particularly described in reference 7--Sensors and Actuators, A21-A23 (1990), pp 273-277, "Monolithic silicon accelerometer" by B. Boxenhorn and P Greiff. This solution suffers from the disadvantage of using a highly doped silicon structure of approximately 10.sup.20 At/cm.sup.3, which reduces the metrological qualities of the transducer. In addition, said transducer operates in torsion.